1. Field of the Invention
The present invention relates to an ignition timing control method and an ignition timing control apparatus for an internal combustion engine, and particularly to an ignition timing control method and an ignition timing control apparatus for an internal combustion engine of so-called dual injection type that includes an in-cylinder injector for injecting a fuel into a cylinder and an intake port injector for injecting a fuel into an intake manifold or intake port.
2. Description of the Background Art
An internal combustion engine of so-called dual injection type is known from Japanese Patent Laying-Open No. 2001-020837. This internal combustion engine includes an in-cylinder injector for injecting a fuel into a cylinder and an intake port injector for injecting a fuel into an intake manifold or intake port. According to an operation state, switching is made between these injectors for use to achieve for example stratified charge combustion in a low-load operation region and homogeneous combustion in a high-load operation region, or these injectors are simultaneously used while the fuel injection ratio between the injectors is changed to achieve homogeneous lean combustion and homogeneous stoichiometric combustion, thereby improving fuel economy characteristics and output characteristics.
Generally, for an internal combustion engine of the fuel injection type, in order to allow proper combustion to occur according to an operation state, a final ignition timing is determined by adding various corrective advance (or retard) values according to an engine state to a basic ignition timing value that is set in advance in association with the operation state and stored for example in a map. Ignition is caused to occur based on the determined final ignition timing for operation.
Regarding the aforementioned dual-injection-type internal combustion engine, because of the difference in injection manner, namely depending on whether the injection manner is the one in which the fuel is injected from the in-cylinder injector or the one in which the fuel is injected from the intake port injector, the temperature of an air-fuel mixture and the state of mixture of the fuel in a combustion chamber could vary. A resultant problem is that, if ignition is simply caused for operation based on an ignition timing value that is set according to an operation state, the ignition timing is improper.
For example, when the fuel is injected from the in-cylinder injector, it is likely that the fuel distribution in the combustion chamber is uneven as compared with the case where the fuel is injected from the intake port injector. If the air-fuel mixture is ignited in the state where the fuel distribution is uneven, combustion proceeds rapidly in a high-fuel-concentration portion and thus the combustion rate of the air-fuel mixture tends to be high. Thus, since the combustion rate of the air-fuel mixture varies depending on whether the fuel injection manner is the fuel injection by the in-cylinder injector or the fuel injection by the intake port cylinder, the ignition timing has to be set appropriately. Otherwise, such abnormal combustion as knocking occurs or the output power (torque) is insufficient.
Further, when the operation is in a transitional state, specifically, when the injection manner is switched from the injection by the in-cylinder injector or from the injection by the intake port injector or the injection ratio between these injectors is changed, the change in injection manner or injection ratio could result in a different port wall temperature and a different inner-cylinder-wall temperature as well as a different amount of fuel sticking to the port wall, the inner cylinder wall and the top wall of the piston as compared with those in a normal state. Accordingly, the temperature of the air-fuel mixture and the air/fuel ratio in the combustion chamber could be over-rich or over-lean. In such a state, if the ignition timing is set to the one determined in a stable state, knocking due to excessive advance or insufficient output power (torque) due to excessive retard for example could occur because of the ignition timing that is different from a proper value, resulting in torque variations.